Magnetic Jets Reveal How Stars Begin Their Final Transformation

John P. Millis, PhD for redOrbit.com – Your Universe Online

In general, main sequence stars die in one of two ways: high mass stars, those several times larger than our Sun, will explode in brilliant supernovae, leaving behind a shock remnant with a dense neutron star or black hole at its center. Lower mass stars like our Sun will transition into a planetary nebula. The outer envelope of the star is expressed into the interstellar medium, and the star’s gravity relinquishes its grip.

But the exact process by which planetary nebulae – which have nothing to do with planets, by the way – evolve and establish their shapes, has been unclear. Now, however, new observations using the CSIRO Australia Telescope Compact Array, may shed some light on this mystery.

Analyzing data from the star IRAS 15445-5449, the team found a high-speed, highly magnetized jet of particles escaping from the dying star. “In our data we found the clear signature of a narrow and extremely energetic jet of a type, which has never been seen before in an old, sun-like star”, says study leader Andrés Pérez Sánchez, a graduate student in astronomy at Bonn University in Germany.

The spectral shape of the jet indicates that it may be laying the framework for the beautiful planetary nebular that will eventually encapsulate the remnant of the star. “What we’re seeing is a powerful jet of particles spiraling through a strong magnetic field”, says Wouter Vlemmings, astronomer at Onsala Space Observatory, Chalmers.

“Its brightness indicates that it’s in the process of creating a symmetric nebula around the star.”

What we may be seeing is the actual transition from normal main-sequence star, to the final stages of its evolution. For the first time, we may be able to actually watch a star die.

“The radio signal from the jet varies in a way that means that it may only last a few decades. Over the course of just a few hundred years the jet can determine how the nebula will look when it finally gets lit up by the star”, says team member Jessica Chapman, astronomer at CSIRO in Sydney, Australia.

There are still questions surrounding this process, however. For instance, is there something special about this particular star that is precipitating the jet formation? Will all stellar mass stars – including our Sun – evolve in a more or less identical manner?

“The star may have an unseen companion – another star or large planet – that helps create the jet. With the help of other front-line radio telescopes, like ALMA, and future facilities like the Square Kilometre Array (SKA), we’ll be able to find out just which stars create jets like this one, and how they do it,” says Andrés Pérez Sánchez.

Image 2 (below): Two older objects, the Calabash nebula (a proto-planetary nebula) and M 2-9 (a young planetary nebula) show how IRAS 15445-5449 (left panel) may evolve in the future. The white bar indicates 0.5 light year. Credits: E. Lagadec/ESO/A. Pérez Sánchez; NASA/ESA & Valentin Bujarrabal; B. Balick, V. Icke, G. Mellema and NASA/ESA

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